As evidenced by the rapid increase in yield following the advent of the Haber-Bosch process, fertilizer N is a necessary component of our cropping systems. By identifying and using practices that maximize fertilizer N uptake efficiency (FNUE), we maximize the amount of fertilizer being taken up by the crop while minimizing the amount left in the soil that can potentially become a pollutant. A major challenge in identifying practices that maximize FNUE is the need to distinguish soil- and fertilizer-N, which can be accomplished using the heavy, stable isotope, 15N. These topics and more will be covered in the presentation.